The present invention relates to weld overlaid stainless clad steels as material for pressure vessels which are used for petrochemical, petroleum refining, coal liquefaction industries and other hydrogenation processes where high-temperature and pressure hydrogen is involved.
Hydrogen having high temperatures and pressures has to be handled in main equipments of a petroleum refining process such as desulfurization and hydrocracking reactors. Reactor vessels used for these hydrogenation services are commonly made of low alloy steels for chromium-molybdenum steels, with stainless steels being typically weld overlaid on the inner surface of the vessels in order to protect the low alloy base steels against corrosive environments such as H.sub.2 S.
In such overlaid stainless clad steels, hydrogen induced disbonding at the interface between the base steel and the cladding is a current material problem, which is accelerated by the increase in the service temperature and hydrogen partial pressures.
It is generally considered that austenitic stainless steels have low sensitivity to hydrogen embrittlement and hence exhibit higher resistance to hydrogen crackings, but, if they are overlaid as claddings on low alloy base steels, hydrogen induced disbonding are much concerned to occur at the interface between the base steel and stainless steel overlaid welds at a shut down condition of the reaction. Two probable reasons for this phenomenon are understood this day as follows: firstly, the weld overlaid austenitic stainless steel forms a different metallic structure near the fusion layer of the welds than when it is used alone, and resulting in coarse grain sizes, and secondly the sensitivity to hydrogen embrittlement increases due to various factors such as carbon migrations from the base metal to the stainless steel during the post weld heat treatments.
With a view to solving those problems, some attempts had been made to improve either the overlay welding materials or the welding conditions but sufficient results are not yet to be obtained.
Nevertheless, with the recent trend to severe service conditions in the hydrogenation processes such as petroleum refining, heavy crude oil cracking and coal liquefaction, there has arisen a strong demand for higher performance base steels and cladding steels with improved resistances against the high temperature and pressure hydrogen environments.
Under these circumstances, the present invention has been accomplished in order to attain the following two objectives regarding with the reliability of reactor vessels used for hydrogenation services:
(1) an improvement in the high-temperature performance of base steel and
(2) an improvement in the resistance of the overlay stainless clad steel to the disbonding caused by hydrogen embrittlement.